The invention relates to process of roughening thermoplastic films. In the present context the term of xe2x80x9cfilmxe2x80x9d also includes plastic products of different thicknesses such as plates, sheets, pellicles, which may optionally be supplied with a base or may be covering layers as well.
The invention further relates to a system for decreasing the slip of films. The technical term xe2x80x9cfilmsxe2x80x9d also means plate-like products of different thickness varying from entirely thin to markedly thick.
It is known that the process for production of thermoplastic films starting from the appropriate mixtures or granules is made so that the unprocessed material is melted or moulded by heating and the desired product is formed in a tool by die-casting or pressing the material through the gap of an extruding die.
The gap does not necessarily determine a flat film, for example in case of hose-like products it can be round shaped as well.
In order to gain its final shape and for the purpose of its stabilisation at leaving the gap of the tool or being taken out of the mould the moulded product is congealed by cooling. Most often the cooling is made by blowing cold air on. After cooling undesired deformation does not occur any more, so cutting to size, winding etc. can take place.
The quality of films"" surfaces is depending mostly on the surface of the tool; in general that is completely plain.
Similarly to the manners as stated above thermoplastic films can be moulded by heat transmission again and may be subjected to further processing.
In connection with the products prepared from plastic films it often creates difficulties that film products, for example folders put on top of each other, are slipping on each other, which can cause falling out of documents, thus the pieces of paper can get dirty and damaged or spoiled. In the same way, in case of plastic sacks instead of the earlier conventional sacks made of textile put on top of each other, when setting up upper sacks, in case of the smallest displacement of the centre of gravity plain surfaces lying on each other can slip on each other to say nothing of shake which takes place in the course of shipment and interplant transport.
Until now slipping of plastic sacks has been tried to prevent in different ways. The most obvious procedure seemed to be to coat sack surfaces lying on each other with adhesive. This solution has raised the problem of choosing the adhesive and the quality of the adhesive. Mostly, when adhesive of an excellent quality was used, it was difficult to pull the sacks apart without the consequence of damaging (deterioration) of the plastic film, while using an adhesive of poor quality due to the insufficient adhesion strength, the task could not be deemed to be solved either.
As an other type of solution it was tried to create antislip surfaces by forming unevenness from the material of the film. According to a typical solution of that kind disclosed in DE 3437414, the film was embossed from the direction of the internal side of the sacks so that bumps were formed on the external side of the sacks. That way the slipping between a film surface with bumps and any other surface not having any bumps is not decreased at all and the slipping between two surfaces having bumps of that kind is somewhat decreased by the contact between the compatible bumps belonging to the opposite surfaces. Those protrusions, however, are not solid and they are, being pressed normally by the opposite film surface, typically plainly lying and archly rounded. Thus the slip decreasing effect is not satisfactory. According to an other solution based on a similar principle grooves were formed in the die casting machine before the gap which created ribs on the films"" surface in the direction of the material""s movement. By this means the problem was solved in part only because the slipping could only be prevented perpendicularly to the line of the aforementioned ribs, the product was able to slip freely in the other direction. Antislip surface on extruded film sheets was tried to create also by forming tips and cavities between the tips on the cylinders forwarding the film by which formations rising from the surface were created. Although these formations were theoretically suitable to prevent slipping in any directions, the required effect it could neither provide.
According to an other solution also known from the prior art unprocessed plastic granules were mixed with extraneous substance which could not be incorporated homogeneously into the basic plastic and humps were formed. However in the course of that kind of method the extruder and the connected passing cylinders were exposed to abrasion, so the tool was getting deteriorated early and besides it was impossible to keep control over the disposal of humps as it would have been required.
A procedure was also applied, where humps were formed on both sides of the film by mixing the raw material with some chemical additive. The drawback of that procedure is its high cost, because the additive, having a high price, was demanded in quite large quantities, and also that both sides of the product were provided with humps, which limited the scope of utilisation.
All of the aforementioned procedures are costly, uneconomical and are not as efficient as it would be required. These methods are used because until now there have been no adequate processes which would solve the problem favourably. A further disadvantage of all these solutions is that humps and formations rising from the surface are covered by the material of the film itself and, although they are subjected to greater abrasion effect, their resistance to abrasion is equal to that of the other parts of the film.
The common disadvantage of the solutions described above is that, due to its extenuation or its having extraneous substance particles implied in its structure, the tensile strength of the product is reduced.
From DE 42 07 210 a process has been known according to which the surface of a thermoplastic plate complement prepared with injection moulding (endproduct) is warmed or partially softened in order to form a molten layer for embedding particles into the same. The plastic surface should be warmed, preferably two times and/or pressing rolls are applied to ensure a safe adhesion/fixing. The specification teaches to provide a deep embedding of the particles into the surface. The material of the said particles may be very different, their geometric features are not critical. Applying that method to any plastic plate components which contain any orientation (coming, for example, from the previous manufacturing process of film blowing) is impossible, because re-heating the plastic component leads to its undesired shrinkage and deformation due to the release of the orientation stress. Thus the application of this method is severely limited. There are solutions known from the prior art dealing with decreasing the slipping of plastic films on each other applying external guiding means or binding elements. GB 2 165 203 discloses a system for preventing teabags from slipping on each other with rigid guiding means perpendicular to the direction of the slipping of the sacks, not being in contact with the contacting/slipping film surface portions. FR 1 334 401 discloses a system in which the outer surfaces of plastic sacks are laminated with rough fabric comprising hard knots decreasing the slipping between these laminated-with-fabric surfaces with the hooking of these textile knots and yarn pieces into each other. That only decreases slip between two surfaces both laminated with rough fabric utilising the co-operation of the two fabric surfaces, actually simulating the slip properties of the earlier conventional jute sacks. The slip between the film surface laminated with the above fabric and another known to the prior art plastic film surface not laminated with the above fabric is therefore not at all decreased, no teaching can be learnt about decreasing the slip in that kind of a contact in the prior art.
The object of the invention is to provide a process of forming suitable antislip films for further use which eliminates the deficiency and drawbacks of the methods known from the state of the prior art without modifications in well known technologies and tools.
The invention is based on the recognition that if particles consisting of the substance of the film and/or other plastic material being able to weld together with that, and having suitable size are brought to the melted film, then the temperature of the particles at their sides looking on the film surface will increase and also themselves will be melted at least partly and will be strongly welded together with the film. Standing out of the surface at least partly, these particles make the surface rough.
According to the present invention the surface of the moulded film, as a semi-finished article, just leaving the gap of the extruder die during the manufacturing process can be roughened.
The substantial quality of the particles used for the creation of roughed surfaces should be selected so that their abrasion resistance and similarly the particle size are suitable for this purpose. If desired, a homogeneous fraction of particles such as particles representing a fraction separated by passing through sieves, are applied to the surface.
The particles may be applied in well ordered or disordered configuration to the entire surface or to specified surface areas such as edges or, if demanded, in spots, repeating the application once or several times depending on the desired pattern. For example the pattern may be formed by using templates or otherwise.
The process according to the present invention is extremely economical, because no special chemical additives and no change of the tool for preparation of the film are needed and similarly, the particular conditions and parameters of the manufacturing procedure are remaining the same. Quite often it is sufficient to rough one side of the product only, however, in accordance with the present invention optionally film made rough either both sides or only one side may be prepared as well.
Thus the essence of the present invention is that particles consisting of the substance of the film and/or other plastic material being able to Weld together with that, and having suitable size and/or demanded abrasion resistance are brought to the surface to be roughened of semi-finished product having sufficient reserved heat content from its manufacturing process to maintain condition appropriate for welding of the thermoplastic film surface to be roughened and the particles are dispersed in suitable closeness and configuration and that condition of the roughened surface is maintained until desired welding process is completed.
The roughed filmsxe2x80x94at least on one side of them on the entire surface or on specified areas of the surfacexe2x80x94comprise protrusions consisting of particles having substance of the film and/or other plastic material being able to weld together with the substance of the film, which are welded into the surface and at least partly standing out of the surface.
The invention further relates to a system for decreasing the slip of films. It is based on the recognition that the connection between a roughed surface and an other, in a given case roughed film surface can be made even more firm if a particular binding element is placed between them.